A belt-type continuously variable transmission (hereinafter referred to as “CVT”) is constituted by winding a belt around an input-side primary pulley and an output-side secondary pulley. Torque of the engine is inputted to the input-side primary pulley, and the output-side secondary pulley outputs torque to wheels. Each of the primary pulley and the secondary pulley includes a fixed pulley and a movable pulley which form a V-groove. Each of the movable pulleys is biased toward the fixed pulley by a primary pulley pressure (hereinafter referred to as “primary pressure”) or a secondary pulley pressure (hereinafter referred to as “secondary pressure”) which is produced by using a line pressure as a base pressure. Accordingly, the belt is sandwiched and held by the pulleys, and a power transmission is conducted between the primary pulley and the secondary pulley.
Because the line pressure is produced by using a discharge pressure of an oil pump as a base pressure, it is known that a level of the line pressure has a great influence on fuel economy. Moreover, in the case that the line pressure is high beyond necessity, there is a possibility that friction in rotating or sliding parts of the transmission is worsened. Hence, a technique to improve the fuel economy has been proposed in which the line pressure is lowered down to a necessary pulley-pressure level so that the discharge pressure of the oil pump is lowered to reduce the friction.
For example, in a line-pressure control device disclosed in Patent Literature 1, a target line pressure is set to be equal to a needed primary pulley pressure or a needed secondary pulley pressure, whichever is greater. That is, a fuel efficiency is enhanced by adjusting the line pressure to a minimum necessary value. Moreover, in this technique, the line pressure is designed not to be uselessly corrected by removing an influence on the line pressure control which is caused due to installation error of a shift actuator or the like, in a HIGH-side region where the primary pressure is higher than the secondary pressure. Accordingly, a worsening of fuel economy which is caused due to excess of the line pressure and a non-attainment of target speed ratio which is caused due to shortage of the line pressure are suppressed.
In the case of Patent Literature 1, the needed secondary pulley pressure is calculated based on an actual speed ratio and an input torque, and a feedback control is conducted according to a difference between the needed secondary pulley pressure and an actual secondary pulley pressure sensed by a sensor. That is, a pressure-reducing valve interposed in an oil passage connected to a secondary pulley chamber is controlled by way of feedback control such that the actual secondary pulley pressure is brought to the needed secondary pulley pressure.
The technique of Patent Literature 1 in which the line pressure is controlled to become equal to the needed primary pulley pressure is particularly beneficial when the speed ratio is in the HIGH side. Also when the speed ratio is in LOW side, the line pressure is controlled to become equal to the needed secondary pulley pressure. Hence, the friction can be reduced over an entire range of the speed ratio. That is, by controlling the line pressure in conformity with higher one of the primary pulley pressure and the secondary pulley pressure, the fuel efficiency can be enhanced while ensuring the minimum necessary level of the line pressure.
Normally in the CVT, an actual pulley pressure is brought to a target value (i.e. target pulley pressure) by way of feedback control. Hence, a pulley-pressure sensor for sensing the actual pulley pressure is provided in order to perform the feedback control. For example, in the case that the line pressure is controlled to become equal to the secondary pressure because the speed ratio is in the LOW side, the fuel efficiency can be enhanced by bringing the actual line pressure to a target secondary pressure. However, the actual line pressure cannot be directly grasped if a hydraulic sensor for sensing the actual line pressure is not provided.
Therefore, the actual line pressure and the target secondary pressure are equalized with each other (hereinafter referred to as “same-pressure control”) by previously controlling the target line pressure. By so doing, a detection value of the pulley-pressure sensor (secondary pressure sensor) for sensing the actual secondary pressure can be used as a value corresponding to the actual line pressure. Hence, a structure in which the detection value of the secondary pressure sensor is brought to the target secondary pressure by way of feedback control is valid.
In the case that the same-pressure control which equalizes the line pressure and the secondary pressure with each other is conducted in execution of the feedback control of the secondary pressure, the target line pressure has only to be reduced from its current value. That is, by reducing the target line pressure, an upper limit value of the secondary pressure is restricted by the actual line pressure when the actual line pressure is lower than a set value of the secondary pressure. If the target line pressure is reduced in this state, the actual secondary pressure is pushed down with the reduction of the target line pressure. By such a process, it is detected that a same-pressure state between the actual line pressure and the actual secondary pressure has been completed, and after that, the actual secondary pressure can be regarded as corresponding to the actual line pressure. Accordingly, a control amount (i.e. feedback correction amount) of the feedback control by which the actual secondary pressure is brought closer to the target secondary pressure is reflected in a control of the target line pressure. By so doing, this line pressure control can adjust the actual secondary pressure to the target secondary pressure while maintaining the same-pressure state between the actual line pressure and the actual secondary pressure.
However, the following problem was discovered. That is, if an upshift is conducted when the secondary pressure is under control of the same-pressure control with the line pressure as mentioned above, a shift shock unintended by a driver occurs. Specifically, when the primary pressure is increased in order to conduct the upshift, the line pressure is increased with this increase of the primary pressure from the same-pressure state realized between the line pressure and the actual secondary pressure. At this time, the actual secondary pressure is temporarily dragged by the line pressure so that the actual secondary pressure upwardly deviates from the target secondary pressure. Afterwards, the actual secondary pressure rapidly drops toward the target secondary pressure so as to cause the shift shock.
One cause of this phenomenon is a structure in which a command value of the secondary pressure continues to be calculated by way of feedback control such as a PI control or a PID control based on a difference (=Target Value−Sensor Value) between the actual secondary pressure (i.e. sensor value) sensed by the sensor and the target secondary pressure (i.e. target value).
That is, even under the same-pressure control, the correction amount continues to be calculated such that the sensor value of the secondary pressure is brought to the target value of the secondary pressure by the feedback control. A value given by adding this correction amount to the target value of the secondary pressure is set as the command value. In particular, when the line pressure is lowered temporarily below the target secondary pressure in order to equalize the line pressure and the secondary pressure with each other in early phase of the same-pressure control, the actual secondary pressure decreases below the target secondary pressure so that the target value and the sensor value become in a deviated state from each other.
During such a period, this deviation between the target value and the sensor value is accumulated by an integral action of the feedback control and is added to the target value of the secondary pressure as the correction amount. Hence, the command value of the secondary pressure is set at a value higher than the target value by the correction amount.
Moreover, there is a case that the same-pressure control between the line pressure and the secondary pressure is terminated when a target primary pressure rises to vary the speed ratio from its current ratio toward an upshift side (HIGH side).
In this case, the line pressure rises because of the rise of the target primary pressure. At this time, the actual secondary pressure does not follow the target secondary pressure but rises by being dragged by the rise of the line pressure. As a result, the actual secondary pressure becomes higher than the target secondary pressure. Then, it is considered that the actual secondary pressure decreases to converge into the target value when an accumulated volume of the correction amount of the secondary pressure is resolved by calculations.
Such a fluctuation of the actual secondary pressure directly affects the speed ratio. In particular, when the speed ratio changes from a LOW-side operating state to a HIGH-side operating state (i.e. upshift) in execution of the same-pressure control so that a magnitude relation between the secondary pressure and the primary pressure is inverted, the speed ratio fluctuates due to the fluctuation of the actual secondary pressure in spite of in-progress of the upshift. In this case, a vehicle occupant sometimes feels a shift shock.